The present invention is concerned with ancillary process and device for the fastening of a tool with any platen press which tool is designed for cutting, creasing or embossing sheet-like material.
A platen press is used for instance for cutting sheet-like material such as paper or cardboard and uses a tool which consists generally a rectangular large sized plate which on one side is provided with cutting devices. This plate is provided in a rigid frame which surrounds its periphery and secures the plate against a lower side of an upper beam of a press. The plate is thus cosidered to be a thin plate which has the tendency to bend between its fastening points situated on the edges of the upper beam and which fastening points consist of the frame surrounding the circumference of the plate.
The bending causes numerous troubles essentially due to the fatigue which has been discussed in both French Patent No. 1,170,528 and in U.S. Pat. No. 3,167,992, whose disclosure is incorporated by reference and which claims priority from a Swiss patent application which was issued as Swiss Patent No. 372,546. It should be appropriate to recall that the bending will occur with the same rhythm as the press during production of cut sheets. In fact, each time the lower beam presses the sheet to be cut against the tool, the whole surface of the tool is pressed against the lower flat surface of the upper beam with the effect to cause the bending to disappear. The bending will then reappear again when the lower beam is moved out of engagement by being lowered. The same appearance will occur again with each cutting operation and at the present speed of which 10,000 sheets per hour can be processed. This means that fatigue fishers and cracks within the elements of the tool can rapidly appear.
In addition, another troublesome problem has shown up of late. Since the above mentioned act of bending brings about the formation of air pockets or strata between the tool and the upper beam or even between the various tool elements with a large flat surface for example between a makeready sheet and its protection or also between the makeready sheet and a rear metal plate as will be seen hereinafter, the volume of air contained within these pockets will be compressed and decompressed with each cutting stroke. In recently constructed high speed machines, the lower beam moves towards the upper beam and away from the upper beam with a very high acceleration and thus the decompression of air occurring during the release of the cutting pressure will end up in a severe release of condensation in the air. Thus, in countries with high rates of humidity, the condensation is likely to cause serious corrosion problems in the press.
Consequently up to now, the theoretical basic solution has consisted in trying tentatively to prevent the tool from being bent.
In order to overcome the bending, the above mentioned French Patent No. 1,170,528 proposes creating a mechanical linkage between certain points of the central part of both the tool and the upper beam. The linkage would be provided with a tightening system to allow pressing the tool against the upper beam with adequate strength. However since the slight bending might still occur between the fastening points, this solution has failed to give full satisfaction. Moreover the mechanical linkage points between the beam and the tool render the assembly and removal of the tool more difficult to a point where they would even hamper the makeready operation. In order to eliminate these drawbacks proposals have been put forth with the view of substituting for the mechanical linkage, electromagnets which are arranged within the holes made on the lower side of the upper beam. However this solution did not turn out to be more satisfactory because in addition to a high cost for the electromagnets, and the cost and inconvenience of providing all of the holes for receiving the electromagnet, the magnets have turned out to be obstructive to the makeready and would not fulfill their function correctly on account of their weak attraction caused by the magnetic saturation of the thin rear metal plate.
Another proposal according to the above mentioned U.S. Pat. No. 3,167,992 was to provide an arrangement of a network of communicating channel situated behind the tool and connected to a vacuum pump in such a way as to apply a suction between the tool and the upper beam of the platen press. The channels, which are provided on a first side of a plate with two parallel sides, are provided with holes all extending to the other parallel side. Appropriate means are foreseen for fitting the channel of the first side against the upper beam. The channels are connected to a vacuum pump however the results obtained with the ancillary fastening method making use of the vacuum achieved in the above mentioned fashion have not proven to be as expected. In fact, the suction power available between the tool and the beam is by far insufficient for the reason which will be discussed hereinafter.